Also, I would call it a theory before I would call it a "fanboy rumor." I certainly haven't heard anyone else say it yet.

Yeah, that's what I meant to say: theory. (Funnily enough, I almost typed "rumor" again just now, and had to backspace when I caught myself.)

And as I've made the case before on these forums, the only possibility is that the existence of the Dark Apprentice is indeed canon. So if LucasArts tosses him aside in the next game, they are gigantic morons. I'd even go on to say that it's possible for the evil clone to outshine Vader as the main villain, but that isn't likely to happen.